An electrochemical sensor consists of a paired sensing and reference electrode combination. In measuring pH or other ion activity it is necessary to use an ion sensing electrode in conjunction with the reference electrode which also makes contact with the system on which measurements are being made. In certain cases the reference electrode can make direct contact with the system which necessitates that the system contain a fixed and known concentration of an ion that can enter into equilibrium with the reference electrode. More generally, the reference electrode is immersed in a solution having a known content of the ion with which it equilibrates and contact of this solution with the system being measured is made via a suitable "salt bridge." This bridge may consist of an extension of the vessel in which contact with the reference electrode is made. A specific ion sensor of the latter type is disclosed and claimed in U.S. Pat. application Ser. No. 491,772, filed July 24, 1974 (now abandoned), and assigned to the assignee hereof.